In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
Currently available cutting elements utilized in drag bits use superabrasive materials such as, but not limited to, polycrystalline diamond (PCD). The superabrasive layer or table is supported by or joined coherently to a substrate, post or stud that is generally made of cobalt tungsten carbide or cemented carbide. Cobalt tungsten carbide is generally selected for the substrate because of its excellent mechanical properties like abrasion resistance and compressive strength.
Bonding the superabrasive layer to the substrate generally occurs during the sintering stage of the superabrasive layer at high-pressure high-temperature (HPHT). Particularly when the superabrasive layer is PCD, the sintered PCD layer is composed of diamond particles with extensive amounts of direct diamond-to-diamond bonding or contact as the major phase. In the interstices between the diamond particles, and to some extent between some of the bonded diamond particles at their boundaries, there is a secondary phase which is also called the metal phase or the catalyst solvent phase. This secondary phase forms a network intermingled with the diamond network. The secondary phase serves as the catalyst or solution for the growth of the diamond-to-diamond bonding. The secondary phase generally includes at least one active metal, for example, but not limited to, cobalt (Co), nickel (Ni), or iron (Fe).
Additional minor phases generally form either in the secondary phase or between the secondary phase and the diamond particles. These phases may include the metal carbides formed during the sintering process. These phases can form isolated islands and/or embed in the secondary phase without clear boundaries.
A process generally used for sintering the currently available cutting elements is the HPHT process, an example of which is shown in FIGS. 11 and 12. Specifically, the process includes adding diamond particles 112 and optional sintering aids 114 to a metal container 110. Then, a carbide substrate 118, generally cobalt tungsten carbide, is inserted into the metal container 110 in contact with the diamond feed 116 including optional sintering aids. The assembly 120 including the container 110, diamond feed 116 including optional sintering aids and carbide substrate 118 is subjected to the HPHT process. During the HPHT process, the binder phase originally present in the carbide substrate will be molten, turned into the liquid solvent phase, and squeezed into the diamond compact due to the high temperature 124 and pressure 122. The flow of the liquid solvent phase is also called sweep due to the fact that the liquid solvent (arrows 126 representing direction of the liquid solvent flow) will form a front face 128 while infiltrating, which carries binder and other materials from the substrate to the diamond feed.
When the diamond is submerged or surrounded by the sweeping liquid solvent phase, the diamond sintering takes place via the liquid-sintering mechanism of solution-transportation-reprecipitation. Here, the diamond-to-diamond bonding is formed and the network of diamond is built. Thus, after sintering, a superabrasive compact 100 is formed having a superabrasive layer 102 and a carbide substrate 104 bonded together at an interface 106. Based on the liquid solvent sweeping from the carbide substrate to the superabrasive layer, the portion of the carbide substrate 104a nearest the interface 106 and the exposed surface portion 102a of the superabrasive layer farthest from the interface 106 contain detrimental effects as explained below.
As mentioned above, the binder from the substrate also carries certain amounts of dissolved species from the substrate into the diamond layer. The amount of the species depends strongly upon the pressure and temperature and the composition of the substrate. Particular species that are carried with the liquid solvent phase include, for example, tungsten and carbon. The dissolved tungsten will react with binder metal and/or carbon from the diamond feed and carbide substrate. Depending on the pressure, temperature, and the composition of the liquid solvent phase, the reaction products might stay in the liquid solvent phase as solid solution species or precipitate out as carbide-based phases after cooling down to room temperature when the process is finished. This liquid solvent phase and other precipitated minor phases remain in the sintered diamond layer in between the grains and form the network of the secondary phase in the diamond layer.
The binder phase of the carbide substrate is primarily the active metal species mentioned above. However, due to use of a Co—WC substrate in the traditional HPHT process, W-C based phases will often be present in the secondary phase in the diamond layer. Many times a phase composition of W, C, and solvent metal M described by the general formula WxMyC and commonly referred to as eta-phase, will form.
One specific eta-phase, Co3W3C, is often detected within the diamond table when enough tungsten from the carbide substrate is dissolved into the liquid solvent phase and reacts with carbon during the HPHT process. This eta-phase is known to be brittle and can be the weak link in the whole composite structure as a crack initiator. Thus, the eta-phase has detrimental effects on the mechanical properties such as abrasion resistance and toughness of the diamond table.
Eta-phase tends to appear at higher sintering temperatures and pressures, which are the conditions often used for high quality diamond compacts to enhance the diamond-to-diamond bonding. Therefore, the traditional HPHT process leads to the choice between desirable HPHT conditions for high quality diamond compact and elimination of the brittle eta-phase that tends to emerge at the desirable HPHT conditions.
In addition to eta-phase formation, the traditional HPHT process has the further disadvantage that the secondary phase for the superabrasive layer comes from the carbide substrate. This phase is not homogenously transferred from the carbide substrate to the superabrasive layer. Instead, the secondary phase comes mostly from the portion of the carbide substrate 104a that is nearest the interface 106. Therefore, during sintering a surface zone of the carbide substrate along the interface 106 becomes depleted of binder such that the metal content in the substrate near the interface is lower than the bulk. Less metal content in the substrate increases the hardness while decreasing the toughness. Because the interface area of the carbide is under maximal axial tensile residual stress, less tough carbide from lower metal content tends to fail easier than carbide with more metal content.
A further disadvantage of the traditional HPHT process is that by sweeping binder from the carbide to the superabrasive layer, the direction of sweep through the superabrasive layer is from the interface 106 towards the exposed surface portion 102a. This generally yields sintered diamond with inferior quality near the exposed cutting portion. This might be tied to sweeping in the traditional direction, where all the impurities or debris in the diamond feed might be swept to the exposed surface portion 102a, which is the working surface of the superabrasive compact.
Several methods have been proposed to reduce or eliminate the eta-phase in the carbide or diamond. As mentioned in U.S. Patent Application Number 2005/0061105, the binder concentration has been controlled to eliminate the eta-phase in carbide composite. Further, international patent application number WO 2008/053430 proposed a method to significantly reduce the eta-phase in the diamond composite by the addition of fine WC particulate into the feed as a dopant at fairly low mass levels prior to sintering. The XRD results confirm the reduced amount of eta-phase in the sintered compact layer. However, none of the prior art solves all of the disadvantages of a traditional HPHT sintering process.